It has become increasingly clear that bacteria are involved in the initiation and progression of periodontal disease. We have focused on one of these periodontal pathogens, A. actinomycetemcomitans, a gram-negative capnophilic bacterium that is considered the major contributor to the etiology of Localized Juvenile Periodontitis (LJP); it has been implicated in adult periodontitis and in cases of endocarditis and other systemic infections as well. Individuals infected with A. actinomycetemcomitans are capable of generating an immune response and both B and T lymphocytes appear to be important. The antigens on A. actinomycetemcomitans that are recognized by B cell antibodies have been well defined, but much less is known concerning the T cell epitopes on this bacterium. Yet the T cell has been implicated both in protection of the host from infection and in the periodontal pathogenesis itself. Careful characterization of the T cell response to A. actinomycetemcomitans is a critical first step in the rational design of a vaccine and towards elucidating a possible direct role for T cells in the pathogenesis of periodontal disease. Towards this end, we propose a novel, T cell hybridoma-based approach in order to begin to dissect the T cell response to A. actinomycetemcomitans. In Specific Aim I, we will compare the T cell repertoires from mice that have been either immunized or infected with A. actinomycetemcomitans. Then in Specific Aim II, we will ask whether the secondary T cell response seen after infection differs from the primary response. In all cases, A. actinomycetemcomitans-specific T cell hybridomas will be generated. Characterization of the T cell antigen receptors expressed by these hybridomas will reveal any evidence of a superantigenic stimulation by this bacterium. In addition, T cell cross-reactivity and specificity for individual proteins will be assessed. The hybridoma approach will provide us with the first opportunity to characterize the T cell repertoire in response to infection with A. actinomycetemcomitans. The information obtained should prove critical in the design of vaccines which may become increasingly important as antibiotic therapies become compromised.